Conventional switchmode converters typically include passive rectifying devices, such as Schottky diodes, through which output or load current flows during operation. In order to reduce conduction losses and improve efficiency, however, a passive rectifying device may be replaced with an active switching device (e.g., a transistor), referred to as a synchronous rectifier. Such a synchronous rectifier may be periodically driven into conduction and non-conduction modes in synchronism with a periodic waveform.
A problem associated with using transistors as synchronous rectifiers is the requirement for properly timed drive signals to control the switching of the synchronous rectifier transistors. For example, improper timing of synchronous rectifiers in DC to DC converters may result in cross conduction through the rectifier at times other than the commutation interval, and/or in reverse recovery currents caused by the conduction of the body diode of one rectifier at the turn-on instant of the second rectifier. In the case of a converter providing electrical isolation between its input and output, this problem is more serious because the synchronous rectifier drives are isolated from the drives of the primary side transistors.
U.S. Pat. Nos. 5,872,705, 5,528,482, 5,303,138 relate to controlling the timing of the synchronous rectifiers in certain clamped converter topologies. Generally, these patents all describe using the voltage across the transformer secondary winding to directly drive the synchronous rectifier that should conduct while the transformer voltage is clamped. For example, U.S. Pat. No. 5,872,705 shows a clamped forward converter in which the flywheel rectifier is driven on in direct response to the secondary voltage being clamped during the reset interval, and is driven off in direct response to the secondary voltage not being clamped and the primary switch that couples the transformer primary winding to a DC power source being on. Such driving and timing control of the synchronous rectifiers is evidently not well suited for topologies that do not have a clamped secondary voltage signal. For example, as may be appreciated, in topologies that do not have such clamping, during the period that a primary switch (i.e., a switch that couples the transformer primary winding to a DC power source) is off (e.g., during the reset interval in a forward converter, or the flyback interval in a flyback converter), the magnitude of the secondary voltage may decrease to a level insufficient to maintain a synchronous rectifier in an on state for the desired time required to provide synchronous rectification and, a fortiori, the associated advantages would be lost. For instance, if the rectifier turns off before the current through it becomes negligible, then the current will flow through the rectifier's body diode, resulting in inefficiencies (e.g., losses) associated with, for example, reverse recovery currents as well as a larger voltage drop across the rectifier than if the rectifier remained on.
U.S. Pat. No. 6,011,703 relates to a self-synchronized gate drive for a synchronous rectifier. The self-synchronized gate drive includes a drive winding associated with the transformer secondary, and the drive winding is coupled to a discharge device and to a drive switch that in turn is connected to the control gate of a second rectifier. The primary winding of the transformer is selectively connected to a DC power source via a primary switch, in a conventional manner. While the primary switch is on, the drive winding voltage drives the drive switch and discharge device such that the discharge device is off and the control terminal of the second rectifier is charged, causing the second rectifier to be on. Alternatively, while the primary switch is off, the drive winding voltage is clamped and drives the drive switch and discharge device such that the discharge device discharges the control terminal of the second rectifier which is thus maintained off synchronously with the voltage on the secondary. It may be understood that if the magnitude of the secondary voltage were not maintained at a sufficient level during the period when the primary switch is off (e.g., the secondary voltage not clamped and the transformer resets before the end of the primary switch off period), synchronous rectification would not occur. Accordingly, as for the aforementioned patents, this patent also discloses using the secondary winding to control rectifier switch timing in a manner that may provide synchronous rectification in a limited range of topologies.
It may be appreciated, therefore, that there remains a need for further improvements and advancements in providing synchronous rectification in switchmode converters. For example, such improvements and advancements would provide sufficient timing accuracy to avoid the deleterious effects of, for example, cross-conduction and reverse recovery currents, while preferably also being well suited for myriad converter topologies (e.g., not limited to topologies that provide a clamped secondary voltage during the off-period of the primary switch).
In various applications, such as in switchmode power supply control, it is desirable to have a pulse width modulation (PWM) controller that has a maximum duty cycle which is not necessarily approximately 50% and which is preferably relatively temperature insensitive.